Plasma arc torches generally include a metallic electrode and a nozzle assembly positioned adjacent the discharge end of the electrode. These torches typically operate in a transferred arc mode in which an arc extends from the discharge end of the electrode through the nozzle to a workpiece. An oxidizing gas is normally used in the torch for improved plasma generation and for facilitating faster and more efficient cutting of the workpiece.
Due to the high voltages required for starting and transferring the arc from the electrode to the workpiece, some plasma arc torches have been started by creating a pilot arc between the discharge end of the electrode and the nozzle assembly. During this starting step, the gas plenum of the torch is often flooded with a non-oxidizing gas so as to reduce the oxidation conditions that would otherwise reduce the effective life of the electrode due to the high voltages that are imposed between the electrode and nozzle assembly. After the torch has been started, the arc between the electrode and nozzle assembly is then transferred to the workpiece. The flow of non-oxidizing gas is also then reduced, and an oxidizing gas such as oxygen is added to the flow of the non-oxidizing gas for improved cutting.
Generally, the aforementioned prior art method of torch starting requires careful control and timing of the gas flow. In some torches, a special torch structure is required. For example, in one prior art torch design, argon flows through multiple annular gas ports positioned between two nozzle members during initial arc starting. After the arc has transferred to the workpiece, some argon flow in the gas ports is terminated and is substituted with a flow of oxidizing gas so that during the transferred torch operation, a reduced flow of argon is mixed with an oxidizing gas. This use of a combination of argon and oxygen, or air, within the torch necessitates simultaneous, complex control over two different gas flows for maintaining proper mixing and operation of the torch. Additionally, use of a non-oxidizing gas such as argon, in combination with an oxidizing gas such as oxygen or air, may result in increased formation of dross, which is undesired.
One prior art torch starting process is described in U.S. Pat. No. 5,017,752, issued to Severance, Jr., et al. on May 21, 1991 and assigned to ESAB Welding Products, Inc., entitled "Plasma Arc Torch Starting Process Having Separate Generated Flows of Non-Oxidizing and Oxidizing Gas." As illustrated schematically in FIG. 2 herein, the prior art apparatus and method as shown in the Severance, Jr. '752 patent includes a torch in which an oxidizing gas such as oxygen (O.sub.2) and a non-oxidizing gas such as Nitrogen (N.sub.2), may be selectively introduced into the torch body via a pair of normally closed solenoid valves V. A gas feed line directs the oxidizing or non-oxidizing gas from the solenoid valves to the gas plenum at the tip of the torch. Thus, as described in the Severance, Jr. '752 patent, the solenoid valves V may be engaged first to introduce a non-oxidizing gas N.sub.2 into the gas plenum of the torch for starting. Thereafter, the solenoid valve V controlling the non-oxidizing gas N.sub.2 may be closed, and the valve V controlling the oxidizing gas O.sub.2 is opened, thereby substituting one gas for the other when the cutting stage is initiated. The respective valves V may also be opened and closed as appropriate to exchange the non-oxidizing gas for the oxidizing gas at the end of a cut and to purge the oxidizing gas from the torch to prepare for a successive starting of the torch to initiate another cut.
One limitation of the prior art apparatus and method shown in FIG. 2 and described in the Severance, Jr., et al. '752 patent is the time delay or lag that is inherent in exchanging (or purging) the gases O.sub.2 and N.sub.2 from the torch. This time delay or lag is due to the volume of gas contained within the tubing and passageways extending between the solenoid valves V and the gas plenum adjacent the electrode and torch nozzle assembly. All of the undesired gas to be purged must be ejected through the nozzle of the torch, which is a time consuming process dependent on the size of the nozzle orifice, the length and volume of the gas tubing, gas passageways and plenum, the rate of flow of new gas into the tubing, passageways and plenum, and the rate of flow of the purged gas through the nozzle orifice.
Often, the size of the nozzle orifice is the limiting factor. For example, in low current torches, typically operating at between 15 and 100 amperes, the nozzle orifice is usually very small. The gas flow pattern through these lower current torches may therefore be restricted, and purging delayed, due to the small nozzle orifice. Consequently, the time required to purge one gas in favor of the other is greater. This problem may be less severe in relatively high current torches (e.g., those torches operating at over 100 amperes, and possibly 150 amperes or higher) which have relatively large nozzle orifices.
An example of the time delay associated with purging in the apparatus shown in the Severance '752 patent is illustrated in FIG. 4 herein. Each of the four graphs in FIG. 4 is plotted concurrently as to time. The various graphs represent, from top to bottom, gas flow at the solenoid valves V where the oxidizing and non-oxidizing gases are exchanged; the arc current initiated by the power supply; gas flow at the torch nozzle; and the cut that is effectuated by operation of the torch.
By comparing the top (valve) gas flow graph in FIG. 4 to the lower (nozzle) gas flow graph, it is readily apparent that a time period, or lag, "A" is inherent when the non-oxidizing and oxidizing gases are exchanged. For example, when the non-oxidizing control solenoid valve V is opened and a quantity of N.sub.2 introduced into the supply tubing, some time is required for the newly admitted N.sub.2 to reach the torch nozzle. The same time delay situation exists when the flow of the non-oxidizing gas N.sub.2 is stopped and the flow of oxidizing gas O.sub.2 is initiated, and again, when the O.sub.2 flow is stopped at the end of cutting cycle and the non-oxidizing gas N.sub.2 is reintroduced into the torch. As previously noted, the amount of the time lag "A" is directly proportional to the length of the gas feed line extending from the solenoid valves V to the gas plenum of the torch, and further, to the rate of gas flow through the feed lines.
While the problem of the time lag "A" might be solved, at least in part, by adjusting the timing of the opening and closing of the solenoid valves V in a predetermined relationship in advance of initiating a new cut or engagement of the arc current, such timing requires careful adjustment, as in the timing of gas introduction found in some prior art apparatus. The need for accurate advance timing also makes the torch apparatus more complex and its operation more difficult. Also, if a torch is operated for cuts that are not of predetermined duration, the inherent time lag following termination of the cutting arc cannot be overcome if the oxidizing gas is to be fed to the torch throughout the cutting step. Such inherent lag may be especially problematic when it is desired to advance rapidly between successive cuts, since the time lag required to completely purge the oxidizing gas from the torch nozzle is the minimum time delay that can exist between the successive cuts. As shown clearly in FIG. 4, the time lag "A" associated with the post-cut flow of the non-oxidizing gas N.sub.2 will exist if the flow of oxidizing gas O.sub.2 continues throughout the end of the cut.
It is therefore an object of the present invention to provide a plasma arc torch in which undesired oxidation is minimized by providing a flow of non-oxidizing gas during pilot arc generation and which further minimizes the lag time associated with introduction of the non-oxidizing or oxidizing gas in the gas plenum of the torch.
It is a further object of the invention to provide a plasma arc torch in which the time delay between successive cuts is reduced for rapid cut indexing.
Another object of the invention is to provide a plasma arc torch in which the time lag associated with exchange and purging of oxidizing and non-oxidizing gases is minimized without resort to complex advance timing of the actuation of gas control valves.
Yet another object of the invention is to provide a plasma arc torch in which pierce quality is enhanced.